High impact polystyrene (HIPS) resins are conventionally prepared by heat-polymerizing a styrene-polybutadiene rubber solution at a high temperature of about 100 to 150° C. or graft-polymerizing using an initiator. Such rubber modified HIPS is a phase that rubber particles are dispersed in the matrix of polystyrene, the rubber particles having the sizes of about 1.5 to 6.0 μm. When an external impact is applied to the HIPS resin, the rubber particles function to absorb the impact. Accordingly, the dispersion, sizes, and size distribution of the rubber particles affect the mechanical properties of the resin such as impact strength, toughness, surface gloss, and heat resistance.
Although conventional rubber modified HIPS resins are widely used in electrical household goods and office equipments housing, they are inferior to the acrylonitrile-butadiene-styrene (ABS) resin in appearance of the molded articles such as gloss and color. As the molded articles with high gloss are needed in various products including sheets for packing containers, the weakness of the appearance character of HIPS has been an obstacle for expanding the use of the resin.
In general, the smaller the sizes of the rubber particles dispersed in polystyrene are, the better the toughness and surface gloss are, but the worse the impact strength is. However, the larger the sizes of the rubber particles dispersed in polystyrene are, the better the impact strength is, but the worse the toughness and surface gloss are.
Therefore, there has been intensive research to obtain preferable rubber modified HIPS resins. Japanese Patent Laid-open No 4-100810 teaches a method to control the sizes of rubber particles using a rubber polymer having two different cis-contents. The resin of the Japanese patent application shows good impact strength but poor surface gloss.
U.S. Pat. No. 4,839,418 to Schwaben et al discloses a thermoplastic molding material consisting of a hard polystyrene matrix and a soft phase which is distributed in the hard phase and has a particle diameter of less than 0.8 μm. Although the molding material has good gloss, it shows a shortcoming of decreasing impact strength.
U.S. Pat. No. 4,146,589 to Dupre discloses a method for preparing improved monoalkenyl aromatic polyblends having a dispersed rubber phase as rubber particles having a bimodal particle size distribution and compositions thereof. In the patent, two different rubber polymers with different shapes and particle size distributions were used to improve the gloss and impact strength. However, the gloss and impact strength were not satisfactory.
European Patent Publication No. 0 337 569 A1 discloses a high gloss, impact resistant rubber-modified polystyrene composition consisting of a polystyrene and dispersed particles of elastomeric polymers, and optionally polydimethylsiloxane and at least one member selected from the group consisting of mineral oil, metallic salts and amides of higher fatty acids based on the total weight of the composition.
The present inventor has developed a rubber modified polystyrene resin composition with high gloss and impact resistance, which is prepared by polymerizing styrene, polybutadiene rubber and a clay material so as to disperse at least two different sizes of polybutadiene rubber particles in the matrix of polystyrene.